1. Field of the Invention
The present invention relates to a control system for an internal combustion engine, and particularly to a control system which performs an engine output control by changing an intake air flow rate and an ignition timing.
2. Description of the Related Art
Japanese Patent Laid-open No. H10-503259 (JP-'259) discloses a control system which performs an engine output control by changing an intake air flow rate and an ignition timing. According to this control system, the intake air flow rate and the ignition timing are set so that the engine output can be increased by an amount of the previously-set torque margin by changing the ignition timing, and the engine output is increased by changing the ignition timing, for example, when the load on the engine increases in the idling condition. By changing the ignition timing to control the engine output, the engine output can rapidly be changed.
A rapid change in the engine output is required when performing the shift-change of the transmission connected to the output shaft of the engine. When performing the shift-up, for example, an output reduction control of the engine is performed in order to reduce the engine rotational speed, and an output increase control is subsequently performed for recovering the engine output.
As shown in JP-'259, the engine output control by changing the ignition timing has good response performance compared with the control by changing the intake air flow rate. Accordingly, it is preferable to perform the engine output control by changing the ignition timing. However, when the engine output is reduced by retarding the ignition timing, a possibility a misfire may occur becomes higher if the ignition timing is retarded beyond the retard limit. Therefore, it is necessary to perform the retard control within the range of the retard limit. However. JP-'259 does not disclose a method of controlling the ignition timing in the retarding direction taking the retard limit into consideration.